Method for determining pressure conditions in a well bore from shale samples

ABSTRACT

A method for determining pressure conditions in a well bore from shale samples obtained from the well bore at different elevations to thereby determine the requirements for the weight of drilling mud to prevent a blowout of the well. The pressure conditions in the well bore are determined by producing a graphical relationship of the salt which is in free solution in water confined by shale samples obtained at different elevations during the drilling of the well.

[451 June 20, 1972 [54] METHOD FOR DETERMINING PRESSURE CONDITIONS IN A WELL BORE FROM SHALE SAMPLES 3,237,094 2/1966 Blackburn et a1 ..l75/50 3,382,933 5/1968 3,409,092 1 1/1968 3,524,346 8/1970 Schmidt 166/250 Primary Examiner-James A. Leppink AttorneyPravel, Wilson & Matthews ABSTRACT A method for determining pressure conditions in a well bore from shale samples obtained from the well bore at different elevations to thereby determine the requirements for the weight of drilling mud to prevent a blowout of the well. The pressure conditions in the well bore are determined by producing a graphical relationship of the salt which is in free solution in water confined by shale samples obtained at different elevations during the drilling of the well.

10 Claims, 4 Drawing Figures [72] lnventor: Harold L. Overton, 5418 Whispering Creek, Houston, Tex. 77017 [22] Filed: Nov. 24, 1969 211 Appl. No.: 879,147.

[52] 11.8. CI ..l75/50, 73/152, 166/250 [51 Int. Cl. ..E2lb 47/00, E2lb 49/00 [58] Field of Search 175/50; 166/250; 73/ 152, 155

[56] References Cited UNlTED STATES PATENTS 2,300,206 10/1942 Clark ..73/ 152 v I WWW/41 mam A E (M: U, n/u/yazra METHOD FOR DETERMINING PRESSURE CONDITIONS IN A WELL BORE FROM SI'IALE SAMPLES BACKGROUND OF THE INVENTION The field of this invention is methods for determining pressure conditions in a well bore using samples of shale from th well bore.

In the drilling of oil wells, it is well known that varying pressure conditions are encountered at different elevations. To prevent a blowout of a well, it has been common practice for many years to keep a column of drilling mud in the well bore of a weight" or more accurately, a density, which is sufficient to overcome the pressure encountered in the well. Various efforts have been made in the past to assist the driller in locating the areas in the well bore of increased pressures, so that the proper mud weight can be used to prevent the blowout, examples of which are found in U.S. Pat. Nos. 3,382,933 and 3,409,092. In US. Pat. No. 3,382,933, conventional well logging techniques are employed during drilling. In US. Pat. No. 3,409,092, bulk density measurements of shale cuttings taken from the well bore are made and trends shown by graphical representations of such bulk density readings at the increasing drilling depths are utilized to indicate pressure conditions in the borehole.

SUMMARY OF THE INVENTION The present invention relates to a new and improved method for determining pressure conditions in a well bore wherein a slurry is prepared from shale cuttings obtained at difierent elevations during the drilling of the well so that from its filtrate, resistivity measurements and/or sodium ion potentials are obtained to provide a measured relationship of the salt which is in free solution in the water confined by the shale samples obtained at the difierent elevations during the drilling of the well. By graphically plotting the measurements obtained, the location and magnitude of overpressure areas in the well may be determined so that the proper weight of drilling mud may be maintained in the well bore during th drilling to prevent a blowout of the well.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a graphical representation of the results obtained using one embodiment of the method of this invention;

FIG. 2 is a graphical representation illustrating another embodiment of the method of this invention;

FIG. 3 is a sectional view of a shale cutting, illustrating the various components thereof which are-of significance in the method of this invention; and I FIG. 4 is a schematic illustration of a sodium potential testing apparatus suitable for use in one embodiment of the method of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drilling of oil wells, it is customary to use drilling mud for maintaining the well under control so as to prevent a blowout of the well. The drilling mud also serves to assist in the lubrication and cooling of the drilling bit, and therefore, the drilling mud is circulated in the well bore, usually down throughthe drill string to the drill bit and the upwardly through the annulus of the well bore to the surface. The return drilling mud carries with it shale cuttings which are normally separated from the drilling mud on equipment known'as a shale shaker. In carrying out the method of the present invention, samples of the shale cuttings which are received at the surface during the drilling of a well are obtained at succesive elevations as the drilling of the well bore progresses downwardly. During the drilling of the first one thousand feet or more of the well bore, the well pressures encountered'are normal pressures and, as will be explained, these pressures are determined using the method of this invention so as to establish a trend line by the graphical representation of the measurements obtained, whereby subsequent excursions or departures from the trend line will be readily apparent at lower depths should they occur. Excursions or departures from the trend line are indicative of the presence and magnitude of overpressure areas in the well bore, and when such overpressure areas are indicated in advance of the actual sand in which the pressure is present, the weight or density of the drilling mud can be increased to compensate for such overpressure and thus to prevent a blowout or loss of pressure through the upper end of the well bore.

Considering the present invention more specifically, a sample of the shale cuttings is obtained at each successive elevation of the well bore for which a determination is desired. The number of elevations at which determinations are made according to the present invention, and the frequency of such determinations, will depend upon the skill and judgment of the operator, but normally, a greater frequency of such determinations will occur at the first indication of an excursion from the normal trend line or normal pressure as reflected by the graph or graphical representations, as will be more evident hereinafter.

For each sample of shale cuttings obtained, the following procedure is followed in the preferred embodiment of this invention, it being understood that variations in the quantities and other conditions of the procedure may be made without departing from the spirit of the invention. First, approximately 50 grams of the sample of shale cuttings is washed so as to free the cuttings of mud. The washed cuttings are then baked at approximately 300 F., while stirring continuously to prevent glazing, until all visible water is removed and the cuttings are dry as indicated by their relatively light color and lack of steam.

Thereafter the dry cuttings are ground, preferably using a mortar and pestle until about 10 grams of fine shale particles are obtained. The fine shale particles should be small enough to pass through a 40 mesh sieve. Next, the 10 grams of the fine dry shale particles are mixed with 10 milliliters of distilled water. The shale particles and the water are mixed and are stirred to form a slurry, with the mixing continuing until the slurry is substantiallyuniform, and then the resistivity of the slurry is measured immediately.

Thereafter, the slurry is filtered, preferably using nitrogen gas to force filtrate at about thirty pounds per square inch pressure, using a No. 50 Whatman filter paper. The resistivity and temperature of the filtrate is then measured. Also, the sodium ion potential of the filtrate is determined, using the apparatus such as illustrated schematically in FIG. 4, as will be more fully explained. The sodium ion potential of the filtrate at successive elevations of the well bore may be plotted to pro- .vide the graphical representation such as shown in FIG. I, wherein the sodium ion potential in millivolts is plotted versus the depth of the well bore, reading in thousands of feet.

Another graphical representation may be obtained using the measurements heretofore described after first calculating the mathematical ratio of the resistivity of the slurry and a resistivity of the filtrate for each elevation. Such ratio is indicated with the letter F" in FIG. 2 of the drawings, and the ratio F at various elevations or depths of the well bore is plotted to obtain the graphical representation such as shown in FIG. 2.

For a better understanding of the significance of the graphical representations of FIGS. 1 and 2, and before considering such graphs more in detail, reference is now made to FIGS. 3 and 4. FIG. 3 is a sectional view of a piece of a shale cutting having a plurality of layers. Typically, a shale cutting would have an upper shale micelle l0 composed of a layer 11 of silicon dioxide (SiO,), an aluminum oxide (A1 0 layer 12 and a third layer 14 of silicon dioxide. A lower shale micelle 20 is composed of layers 21, 22 and 23 which would normally be silicon dioxide, aluminum oxide and silicon dioxide, respectively.

The micelles l0 and 20 are above and below the pore or pocket 25, which is of capillary size. Such pore or pocket 25 is closed on its sides by micelles 30 and 31. Its ends are closed by micelles also (not shown) so that the pore or pocket 25 is completely enclosed. It has been found that when there is a solid stress on the shale cutting down in the well bore prior to its having been cut and removed and circulated to the surface, the stress drives water out of the pore 25 and it causes sodium ions which are indicated as being along the surfaces of the shale by the dotted lines 40 into the water within the pore 25. On the other hand, when a pressure condition exists within the pore 25 which is above normal, i.e., an overpressure, the fluid pressure drives the sodium ions onto the shale so that there is a greater sodium ion concentration on the shale surface than in the water within the pore 25 under such overpressure condition. Such relationship is of extreme importance with respect to this invention, because the measurement of sodium ion con centration of the filtrate, and the relationship of the resistivity of the slurry to the resistivity of the filtrate are indications of the salt or sodium ion which is bound on the surface of the shale as compared to the sodium ion or salt which is in free solution in the water confined in the pore 25 of the shale cutting, and such relationship is therefore indicative of the pressure condition at the elevation in the well from which such shale cutting was obtained.

Various types of apparatus may be utilized for testing the sodium potential of the filtrate obtained in the procedure outlined above, and therefore, the form of apparatus illustrated in FIG. 4 is schematically shown by way of example only. Thus, the apparatus of FIG. 4 includes a container 50 for receiving the filtrate 51 which serves as the electrolyte in the testing apparatus. A reference electrode 52, preferably a standard calomel reference electrode having a conductive tip 52a, is inserted in the electrolyte. A standard solution is provided in the sodium ion electrode 55, which has a membrane 56 formed of glass or other suitable material which is specifically sensitive to sodium ions. The electrodes 52 and 55 are connected to a potentiometer 60 which reads the potential difference between the electrodes, and such potential difference has been designated A E" on the bottom of the graph of FIG. 1.

Thus, in both embodiments of the invention heretofore described, measurements are made which are indicative of the amount of the sodium ion which is present in the filtrate for each sample. In the form of the invention illustrated in FIG. 1, the sodium ion potential is measured and is graphically represented for the successive elevations of the well bore being drilled. With respect to the graphical representation of FIG. 2, the sodium ion concentration is actually measured by measuring the resistivity of the filtrate so that the salts present in the filtrate are all functioning to produce such information, although the information is essentially that of the sodium ion concentration in the filtrate. Also, as noted above, the resistivity of the slurry is 'also obtained so that such resistivity of the slurry is an indication of the salts which are bound to the shale. By plotting the ratio of the resistivity of the slurry to the resistivity of the filtrate at different elevations during the drilling of the well, the effect of changes in the location of the sodium ion concentration with respect to shale cuttings at different elevations can be readily observed, and therefore, the changes in the pressure conditions of the well related to the depth of the well are likewise indicated.

Thus, in FIG. 2, a portion of a typical graph is shown, wherein the depth is shown in feet from 10,200 feet to 11,600 feet. The normal trend is indicated on such graph in FIG. 2 by the dash line. The normal trend is established at the outset of the drilling, usually within the first 1,000 feet of drilling, as previously explained, because in the early stages of drilling at the higher elevations, the pressures encountered are generally normal. In FIG. 2, an extreme excursion of the graph is shown at point 70, which represents a reading of approximately 0.65 on the lower scale representing the ratio of the resistivity of the slurry over the resistivity of the filtrate. Such excursion 70 indicates to the operator that there is an overpressure area being encountered, and such overpressure area is indicated on the graph above the elevation at which the sand having the gas creating such pressure in the well is located. Therefore, the

detection of the overpressure is obtained so as to provide an indication of impeding high formation pressure before drilling into such high pressure. This permits the driller to change the density or weight of the drilling mud to compensate for the anticipated increase in pressure within the well bore so as to prevent a blowout of the well.

The chart or graphical representation shown in FIG. 1 was made on the same well, using samples of shale cuttings at substantially the same elevations as were used in preparing the graphical representation of FIG. 2. The correlation of the graphs of FIGS. 1 and 2 can be seen by a comparison of the excursion 70 in FIG. 2 as compared to the excursion 71 in FIG. 1. Thus, the point 71 is only one of a number points or readings of the sodium potential of the filtrate obtained using the method of this invention which reflects that there is an overpressure in the area of the well bore at approximately 1 1,1 50 feet.

At point 72 in FIG. 2, another prominent excursion or departure from the normal trend line is evident, and this closely corresponds with a point 73 on the graphical representation shown in FIG. 1. The vertical span of the excursions 70 and 72 compared with the vertical span of the excursions 71 and 73 indicate that both graphs are revealing to an operator a relatively wide band of overpressure at the elevations or depths in the well bore indicated. It is significant to note that with respect to both of the graphs (FIGS. 1 and 2), not only the presence of the overpressure has been indicated, but the relative magnitudes of the overpressure in the various areas are also indicated so that the operator can judge the density or weight of the drilling mud to be employed to prevent a blowout.

The foregoing disclosure and description of the invention are illustrate and explanatory thereof, and various changes in the size, shape, and materials as well as in the details of the illustrated construction may be made without departing from the spirit of the invention.

I claim:

1. A method of determining the presence and magnitude of overpressure areas in a well bore during drilling, comprising the steps of:

at successive elevations in the drilling of a well, forming a slurry from'a sample of shale cuttings brought to the surface during the drilling of the well; filtering the slurry from each sample to obtain a filtrate; making a measurement which is indicative of the amount of the sodium ion present in the filtrate for each sample;

preparing a graphical representation of said measurements to obtain a trend line for normal pressure conditions and to show excursions therefrom indicative of the presence and magnitude of overpressure areas in the well bore.

2. The method set forth in claim 1, wherein said measurement which is indicative of the amount of the sodium ion is obtained by:

disposing said filtrate as the electrolyte in an electrical potential testing apparatus with a reference electrode and a standard sodium ion electrode; and

obtaining a reading of the electrical potential difference between said electrodes.

3. The method set forth in claim 1, wherein said measurement which is indicative of the amount of the sodium ion is obtained by:

measuring the resistivity of said filtrate.

4. The method set forth in claim 3, including:

measuring the resistivity of said slurry;

determining the ratio of the resistivity of the slurry over the resistivity of the filtrate; and

plotting said ratio for each elevation for which said ratio is determined to obtain said graphical representation.

5. The method set forth in claim 1, wherein said slurry is formed for each sample by steps, including:

washing the sample of shale cuttings substantially free of mud;

baking the washed shale cuttings until all visible water is removed;

grinding the dry cuttings into fine particles; and

thereafter mixing the finely ground particles with distilled water.

6. The method set forth in claim 5, wherein said measurement which is indicative of the amount of the sodium ion is obtained by:

disposing said filtrate as the electrolyte in an electrical potential testing apparatus with a reference electrode and a standard sodium ion electrode; and

obtaining a reading of the electrical potential difference between said electrodes,

7. The method set forth in claim 5, wherein said measurement which is indicative of the amount of the sodium ion is obtained by measuring the resistivity of said filtrate, and includmg:

measuring the resistivity of said slurry;

determining the ratio of the resistivity of the slurry over the resistivity of the filtrate; and

plotting said ratio for each elevation for which said ratio is determined to obtain said graphical representation.

8. A method of determining the presence and magnitude of overpressure areas in a well bore during drilling, comprising the steps of:

at successive elevations in the drilling of a well, a physically determining the relationship of salts bound on the surface of shale cuttings obtained at each such elevation to the salts in free solution in water confined by the shale; and

preparing a graphical representation of such determinations related to the elevation in the well for each such determination to obtain an indication of the presence and magnitude of overpressure areas in the well bore.

9. The method set forth in claim 8, wherein the relationship of the salts bound on the shale surface to the salts in the free water is obtained using the filtrate from a slurry formed from each sample of shale cuttings by steps, including:

measuring the resistivity of slurry;

measuring the resistivity of said filtrate;

determining the ratio of the resistivity of the slurry and the resistivity of the filtrate; and

plotting said ratios to obtain said graphical representation.

10. The method set forth in claim 8, wherein the relationship of the salts bound on the shale surface to the salts in the free water is obtained using the filtrate from a slurry formed from each sample of shale cuttings by steps, including:

disposing said filtrate as the electrolyte in an electrical potential testing apparatus with a reference electrode and a standard sodium ion electrode; and obtaining a reading of the electrical potential difference between said electrodes. 

1. A method of determining the presence and magnitude of overpressure areas in a well bore during drilling, comprising the steps of: at successive elevations in the drilling of a well, forming a slurry from a sample of shale cuttings bRought to the surface during the drilling of the well; filtering the slurry from each sample to obtain a filtrate; making a measurement which is indicative of the amount of the sodium ion present in the filtrate for each sample; preparing a graphical representation of said measurements to obtain a trend line for normal pressure conditions and to show excursions therefrom indicative of the presence and magnitude of overpressure areas in the well bore.
 2. The method set forth in claim 1, wherein said measurement which is indicative of the amount of the sodium ion is obtained by: disposing said filtrate as the electrolyte in an electrical potential testing apparatus with a reference electrode and a standard sodium ion electrode; and obtaining a reading of the electrical potential difference between said electrodes.
 3. The method set forth in claim 1, wherein said measurement which is indicative of the amount of the sodium ion is obtained by: measuring the resistivity of said filtrate.
 4. The method set forth in claim 3, including: measuring the resistivity of said slurry; determining the ratio of the resistivity of the slurry over the resistivity of the filtrate; and plotting said ratio for each elevation for which said ratio is determined to obtain said graphical representation.
 5. The method set forth in claim 1, wherein said slurry is formed for each sample by steps, including: washing the sample of shale cuttings substantially free of mud; baking the washed shale cuttings until all visible water is removed; grinding the dry cuttings into fine particles; and thereafter mixing the finely ground particles with distilled water.
 6. The method set forth in claim 5, wherein said measurement which is indicative of the amount of the sodium ion is obtained by: disposing said filtrate as the electrolyte in an electrical potential testing apparatus with a reference electrode and a standard sodium ion electrode; and obtaining a reading of the electrical potential difference between said electrodes.
 7. The method set forth in claim 5, wherein said measurement which is indicative of the amount of the sodium ion is obtained by measuring the resistivity of said filtrate, and including: measuring the resistivity of said slurry; determining the ratio of the resistivity of the slurry over the resistivity of the filtrate; and plotting said ratio for each elevation for which said ratio is determined to obtain said graphical representation.
 8. A method of determining the presence and magnitude of overpressure areas in a well bore during drilling, comprising the steps of: at successive elevations in the drilling of a well, a physically determining the relationship of salts bound on the surface of shale cuttings obtained at each such elevation to the salts in free solution in water confined by the shale; and preparing a graphical representation of such determinations related to the elevation in the well for each such determination to obtain an indication of the presence and magnitude of overpressure areas in the well bore.
 9. The method set forth in claim 8, wherein the relationship of the salts bound on the shale surface to the salts in the free water is obtained using the filtrate from a slurry formed from each sample of shale cuttings by steps, including: measuring the resistivity of slurry; measuring the resistivity of said filtrate; determining the ratio of the resistivity of the slurry and the resistivity of the filtrate; and plotting said ratios to obtain said graphical representation.
 10. The method set forth in claim 8, wherein the relationship of the salts bound on the shale surface to the salts in the free water is obtained using the filtrate from a slurry formed from each sample of shale cuttings by steps, including: disposing said filtrate as the electrolyte in an electrical potential testing apparatus with a reference electrode And a standard sodium ion electrode; and obtaining a reading of the electrical potential difference between said electrodes. 